The invention relates to brake pressure control apparatus for an all wheel drive vehicle having two brake circuits, each controlling two wheels and including ABS and ASR. Each circuit includes inlet and outlet valves at each wheel for increasing and decreasing brake pressure, a self priming pump for returning brake fluid released by the outlet valve, a changeover valve between the master cylinder and the point where the fluid is returned by the pump, a check valve in parallel with the changeover valve, and a loading valve connected between the inlet of the pump and a point between the master cylinder and the changeover valve.
In prior art apparatus of this type the brake pressure modulation in the event of anti-slip control action is performed by means of the anti-lock system valves; during the anti-slip control the changeover valve is not open and the loading valve is open. If the driver wishes to brake while anti-slip control is running it can happen that, since all of the inlet valves for the pressure modulation are closed, he finds he has a hard brake pedal and achieves no braking action. As a remedy for this, the brake light switch signal is used to shut off the anti-slip control.